The invention concerns a device for indirect digital front and back side printing of multicolored pictures onto sheets using single shot and single pass methods.
Packaging and label printing are currently growing markets. The packaging market is expected to double within the next five years through the influence of Eastern Europe, South-East Asia and China, wherein plastic materials, sandwich materials and metallized substrates will be increasingly used. The worldwide turnover with packaging printing machines is about one billion Euros (Deutsche Drucker No. 4 of Feb. 6, 2003).
The packaging market poses the highest demands concerning printing and finishing quality. In jobbing (commercial), almost everything is printed with standard process colors, optionally extended by a customer-specific pantone color. In packaging printing many more pantone colors are used, either exclusively or as a supplement to the process colors.
Conventional sheet-fed offset machines are classified by the maximum printable sheet format in accordance with format classes with the following variants:
Small format 353 × 500 (B3)Semi-format 500 × 707 (B2)Medium format 707 × 1000 (B1)Large format1000 × 1414 (B0)
In conventional sheet printing machines in accordance with the offset or book printing method, picture-carrying plates are used in dependence on the format class, which must be exchanged when the motive or order changes. The illustrating or plate cylinders comprise a tensioning channel, which is also format-dependent, and mostly comprise demanding semi-automatic plate exchange systems.
For printing, it is standard to add a particular customer-specific color to the four process colors cyan, magenta, yellow and black (C, M, Y, K). The CYMK color space is likely to preclude readjustment of the pantone reference value. To obtain a larger color range for multi-color printing, complementary red, green and blue (R.G.B) are additionally increasingly used for the 7-color HIFICOLOR system or the additional colors orange and green are used for 6 color hexachrome systems. This is advantageous in that 95% of the pantone colors can be printed without requiring the hitherto associated time-consuming cleaning of the printing mechanism for a new order. This is also confirmed by the increasing application of sheet-fed offset machines with 8 and 10 printing mechanisms, not only for double-face 4-color printing but also for matrix printing with additional colors for this so-called High-Fidelity print.
The color is transferred indirectly from the printing plate to the material to be printed via the (exchangeable) rubber blanket thereby compensating for unevenness of the material to be printed. Surfaces and matrix points are transferred almost as if the material to be printed had an ideal flat surface which permits processing of a large range of printing materials.
This also prolongs the service life of the illustrating cylinders, the transfer band or the transfer drum, since they are not in direct contact with the abrasive surface of the printed material.
The system is very precise (±0.1 mm) since the substrate sheets are aligned through side and front lay marks when the system is stopped.
The printed sheets are transferred from the feeder pile to the first printing mechanism, from printing mechanism to printing mechanism and from the last printing mechanism to the delivery pile using gripping technology which is integrated in the counterpressure cylinder channel or in a chain carrier, in dependence on the format. This means that the separation between a gripper system and the neighboring gripper system is always equal to the maximum printing format in the peripheral direction.
Flexo lacquering mechanisms with fixed format are increasingly integrated in printing machines, since application of a lacquer layer considerably increases the quality of the prints, e.g. protects the printed material and improves further printing processing or e.g. spot lacquering for optical effects.
A further development concerns application of a primer (with primer in the Flexo method) before and after printing e.g. for printing plastic materials with hybrid printing systems, i.e. the combination of different printing methods in one printing machine (U.S. Pat. No. 6,443,058 B1).
NIP methods for personalization (DE 100 47 040 A1), punching units (DE 101 47 486 A1) for further processing, embossing units for haptic effects (Look and Feel) and Inline Finishing (EP 80 929 091 A1) e.g. for folding have recently been integrated in the chain of process consolidation. The above-described combination of high-quality printing, finishing and further processing methods also requires relatively demanding drying systems which results in machine lengths of up to approximately 35 meters, as reported in Druckspiegel No. 5, May 2002 SM Type CD 102 LY-6-LYYLX of the company Thomas in Gelsenkirchen as well as by COMPRESS Magazin on Apr. 6, 2003, concerning a KBA Rapida 142 sheet-fed offset machine having a format of 102×140 and a length of 37.5 meters in Illinois (USA). This machine comprises 7 offset printing mechanisms, one lacquer printing mechanism, intermediate drying and last printing mechanism, including a turning device. These highly demanding machines require i.a. expensive automation and drive concepts with e.g. double drives having toothed wheel tension and cardan shafts as well as several intermediate and final dryers (DE 199 12 309 A1). Without such devices, this technology could not be mastered.
Sheet turning systems are also often integrated in the sheet printing machines to print both the front and the back sides in one process.
For the above-mentioned reasons, a great amount of space is required which poses substantial problems, i.a. to a desired one-man operation, and involves new investment for the extension of the premises. In conventional applications, compact machines of satellite structure are used for small formats, with the number of printing mechanisms being limited to 4. For small, semi-, medium and large formats, modular structures for series construction are therefore frequently used which have their own construction module for each printing mechanism.
For both conventional machine concepts, format-related conventional offset or letterpress plate cylinders having tensioning channels are used. FIGS. 15 and 16 show that the use of 2×7 printing mechanisms of half-size portrait (with 345 mm diameter) would produce ergonomically unrealistic dimensions with this concept and for modular as well as satellite structures, additional modules, e.g. roll unwinding or for finishing or for further processing, are also unrealistic.
In view of the technical effort of conventional printing technologies, and due to the fact that one expects, due to the influence of “POD” (print on demand) or just in-time production, that 90% of all jobbing and a considerable part of packaging printing orders will involve less than 5000 sheets, it becomes clear that other printing machine concepts must be invented to ensure economic future production.
For digital printing machines, a nearly offset-like quality with maximum flexibility is generally obtained in prior art, since each sheet can be continuously printed with another motive if required without losing time for adjustments and without having to change plates. To optimally utilize the data processing speed, the digital printing machines are designed for portrait format processing. The digital printing method requires no demanding radiation and dryer systems, thereby facilitating processing consolidation with additional processing steps. For this reason, the digital printing method is well suited for printing small runs of small formats (currently max. A3 format, approximately 330×460 mm).
Most digital printing machines have paper transport systems e.g. using transport bands (DE 195 36 309 A1). Grippers are not used for transfer of the sheet (except for WO 96/17277). This limits the precision of the color register required (±0.01 mm) and the feed register (±0.1 mm). The tolerances of the feed and transfer passers (color register) are generally larger by approximately a factor of 2 to 4 compared to printing methods using conventional feeding and gripper technology such as e.g. sheet-fed offset printing. These large tolerances (image drift) in digital printing require complicated systems to compensate for, or to attempt to compensate for, these tolerances (image drift) in inline finishing.
More digital printing machines must be used which are suitable for the POD market. The limited technical features preclude use in the graphics industry (Report Pira International Ltd. 2002 ISBN 185824641). Digital machines without grippers are mainly limited by the maximum format size, production speed, flexibility of the material to be printed and feed passer.
It is therefore the object of the invention to develop a new generation of printing machines to meet the new market demands for maximum quality with minimum copies for POD and just in time systems, wherein the advantages of the conventional sheet-fed offset technology and new digital technology must be utilized to ensure future economical production. The requirements are listed below:    1. 1 to 7 digital printing mechanisms with upstream cleaning station for hexachromes or Hifi color print, using single shot and single pass methods;    2. integrated finishing with protective lacquer (100% of the packagings require protective lacquer on one side) and alternatively full-surface lacquer finishing for jobbing printing mechanisms and/or special spot effect lacquer (approximately 20 to 30% of the orders of a jobbing print are lacquered);    3. the possibility of personalization or printing of variable data;    4. indirect print through exchangeable rubber blankets to compensate for unevenness in the material to be printed;    5. a format class which is larger than the small format (approximately 36×50 cm), preferably a 50×70, 70×100 format;    6. the possibility of printing plastic material and sandwich substrates;    7. full-format duplex print on front and rear side without turning and at full production speed (for jobbing prints approximately 90 duplex is printed and for packagings approximately 5 to 10 is printed and/or finished. (e.g. rear side print with instructions, safety features or protective lacquer or coating for the inside of the packaging);    8. uniform printing methods for printing, coating and finishing to permit automation and reduce the requirements for operator skill;    9. high-quality sheet alignment and gripper sheet transport systems for feed and transfer passers, similar to sheet-fed offset, with minimum gripper change or gripper transfer;    10. high-quality arrangement of the printing cylinders preferably with counterpressure cylinder of twice the periphery and delivery arrangement in the so-called 7-o'clock position to permit perfect undistorted printing (tangent function);    11. high-quality arrangement and drive of the printing cylinders for extremely high register accuracy on one side for multi-color print and also between front and rear side print.    12. Straight (minimized) sheet guidance for maximum flexibility of the material to be printed and for separating the sheet from the rubber blanket cylinder with minimum force;    13. Stability with minimum operating oscillation for optimum printing quality despite tensioning channels in the intermediate or rubber blanket cylinder for use of novel liquid toner, which requires more pressure than dry toners;    14. Sheet guidance without blotting;    15. Good accessibility of the individual machine elements;    16. Inline finishing with exact register through gripper transfer such as e.g. hot foil embossing and/or punching and/or piling or inline folding or inline book binding;    17. Complete utilization of the synergy of common parts, modules and software of a family of printing machines for inexpensive mass production;    18. One size for one-man operation, preferably one machine length of a maximum of approximately 7 m and a machine height of a maximum of approximately 2.75 m.
U.S. Pat. No. 5,016,056 discloses principal prior art which is not based on indirect print transfer via an intermediate carrier or rubber blanket cylinder. Printing is effected directly from the illustrating cylinder. This lacks the advantages of indirect printing via rubber-coated intermediate cylinders, in particular, advantageous printing on uneven substrate surfaces and the associated increased flexibility with respect to the material to be printed.
CH 116 828 describes conventional offset printing mechanisms with format-dependent plate and rubber blanket cylinders which therefore both have tensioning channels. A 2×7 color printing machine of medium format is excessively large for both a satellite as well as a modular arrangement (FIGS. 15 and 16). Change of motif requires demanding plate change and in most cases rinsing of the printing mechanism for other customer-specific pantone colors.
Neither does DE 100 47 040 A1 discuss digital printing mechanisms, rather offset printing mechanisms which are digitally exposed online, however, using conventional plate and rubber blanket cylinders which are format-dependent and have the above-mentioned disadvantages.
DE 21 15 790 A1 also describes conventional offset and/or letterpress printing i.e. with format-related plate cylinders having tensioning channels and the above-mentioned disadvantages.
DE 199 12 309 A1 provides an example of a machine of modular structure (U.S. Pat. No. 6,443,058 B1) which is excessively long (approximately 25 m). DE 100 47 040 A1 suggests a satellite arrangement with only 4 printing mechanisms and a connected printing mechanism with coupling means required therefor. This machine disadvantageously requires a second passage for the second print (approximately 90–95% of the jobbing prints are front and back side prints) and is also not suited for 7 color print with subsequent finishing.
DE 21 15 790 A1 describes a construction or printing machine concept, which permits duplex printing in one process but which is a combination of format-dependent plate imaging systems combined with conventional format-dependent rubber blanket cylinders. This construction does not permit integration of up to 2×7 printing mechanisms or even further modules for coating without creating unacceptable handling and engagement problems (FIG. 15). This factor is of particular importance since conventional digital printing mechanisms are based on portrait printing (i.e. printing of a page in a vertical orientation), in contrast to landscape printing in conventional sheet-fed offset printing (i.e. printing of a printed page in a horizontal orientation). Moreover, additional space must be reserved for format-related illustrating cylinders or drums for access, e.g. for plate and/or rubber blanket replacement. For this reason, the maximum satellite arrangement is considered to be 4 printing mechanisms (DE 43 03 796 A1). CH 116,828 also discloses plate and rubber blanket cylinder constructions with bound format for tensioning imaging plates and rubber blankets. The format-related technology does not permit extension to 2×7 printing mechanisms with cleaning systems or even additional mechanisms for finishing.
Conventional satellite printing machines (WO 01/39976 A1) do not take into consideration the above-mentioned requirements of digital printing with regard to format-independent illustrating cylinders. Illustrating cylinders of fixed format are used which therefore cannot utilize the considerably compact construction of the inventive machine.
U.S. Pat. No. 5,016,056 discloses sheet transport without formatted gripper systems and avoids use of highly precise sheet gripper transport systems with projecting gripper backs which would damage the illustrating cylinder, by using a vacuum strip which holds the sheet on the feed side without protruding. The production tolerances of the feed passer can be expected to vary by a factor of between 2 and 4—larger for the sheet feed and transport system than for printing methods using conventional gripper technology. Moreover, such systems without grippers are limited with respect to the flexibility of the material to be printed, the sheet format and the sheet thickness of the printing system. The ends of the sheet are also held by vacuum. This is disadvantageous in that only sheets of a fixed peripheral length can be printed (“secures the ends of a receiving sheet”).
DE 195 36 359 A1 discloses an endless transport without gripper systems, wherein feed and transport passer tolerances must be expected which are a factor of 2 to 4 times larger than for sheet feeder and transport systems using conventional gripper technology.
CH 116,828 provides duplex printing in one step but only at half the production speed since “a sheet must be supplied at least after every second rotation”.
In known satellite printing machines with gripper transport devices according to DE 43 03 796 A1, the number of rubber and plate cylinder pairs disposed about a printing cylinder is limited to four due to the need for access to the printing mechanisms. Front and back side printing (duplex print) therefore require sequential arrangement of two printing mechanisms or twin stations which must be connected via a turning unit as also provided e.g. in U.S. Pat. No. 5,660,108 and DE-PS-435 902.
There are various conventional concepts of digital printing mechanisms for duplex printing (front and rear side printing) e.g. via a turning pocket (U.S. Pat. No. 5,552,875) (which includes the risk of distortions, paper jamming, damage, halved productivity, for limited thicknesses and is not that precise), twin installation (associated with inflexibility, large investment and many gripper transfers) or systems with half the width or half the circumference.
Turning systems are known for sheet printing machines (DE 298 07 663 U1) for printing the first and second side of the sheets (recto verso). These systems are demanding, render the machine inflexible due to their fixed position, are expensive and require a white edge (gripper edge) on both sides of the sheet. Moreover, the registering sheet guidance (turning passer) is extremely difficult and leads to inaccuracies. It also limits the flexibility of the printing material with regard to substrate thickness.
For applications which only require occasional duplex printing, it is, however, feasible to integrate a conventional turning drum system, wherein the above-mentioned disadvantages must be accepted.
EP 819 268 B1 discloses a digital printing mechanism using the so-called multi-pass system, wherein the intermediate cylinder passes several times through the same printing gap and transfers the multicolored image formed on the rubber blanket cylinder onto the printing material when the sheets are supplied in cycles during the so-called single shot procedure. The associated efficiency is therefore very poor. The multiple transfer on the intermediate cylinder could have negative effects on the register accuracy e.g. through slight bulging/speed differences during multiple passage of the printing gaps. The illustrating cylinder is designed for replaceable plates or cylinder milling and has a tensioning channel for tensioning or holding the plate. The so-called photo imaging plate must be regularly replaced due to wear. This construction is bound to a format and for this reason cannot receive more than 4 printing mechanisms when used in a satellite construction due to access needs (replacement of plate and rubber blanket) (DE 43 03 796 A1).
U.S. Pat. No. 6,363,234 B2 discloses a satellite construction with format-related printing mechanisms/print engines which are limited to a maximum of 4 for access reasons. A special turning technique cuts the productivity in half.
The photo conductor drum or illustrating cylinder 52 (FIG. 5), on which the toner image is formed from the charge image derived from the optical image, is the central component in the electro-photographic process.
FIG. 1 shows how the partial colors formed by the individual photo conductor drums S are collected on the rubber blanket segments before being transferred to the printing material (single shot).
Partial colors can also be transferred onto a conducting rubber-like silicon transfer band (FIG. 22) or transfer drum (FIG. 23) or onto a common photo conductor drum and then onto an intermediate cylinder having rubber blanket segments to transfer the collected partial colors to the printed material.
There are a plurality of digital printing techniques for transferring variable data with color onto the material to be printed. The best known methods are inkjet, thermo transfer, thermo sublimation, electro photography, magnetography, ionography and direct imaging technology (U.S. Pat. No. 3,846,840).
Digital printing with gripper sheet transport is not possible, since the transfer band would be damaged by direct contact with the projecting grippers, similar to the photo conductor drum.
It would be feasible to design the rubber-like silicon transfer band or transfer drum or common photo conductor drum to be compressible in order to compensate for unevenness of the substrate, such as for a rubber blanket. If these parts are delivered with format-dependent recesses for the gripper backs, they could be disposed directly on a counterpressure cylinder with gripper transport system (not shown). Bands are, however, not as precise as drums and are moreover limited in length and maximum production speed.